Arrangement for changing the direction of polarization of a polarized beam of light

ABSTRACT

A method for influencing a light beam by applying an electrical signal to cause phase modulation, amplitude modulation or deflection of the light beam using an electro-optic crystal made of a compound having a cation and an anion and containing a thiocyanate complex in the anion. The compound has two different metals which are selected from Groups I and II of the periodic system of elements. Preferably one of the metals is mercury and forms with the thiocyanate complex the anion complex having a formula of (Hg(SCN)4). The other metal is preferably selected from a group consisting of zinc, cadmium, copper and gold. The electro-optic crystal is used with a polarizer to phase modulate a light beam in accordance to the voltage applied to the electrodes of the crystal or the electro-optic crystal can be used to amplitude modulate a light beam by being placed between a pair of polarizers. The electro-optic crystal can be utilized between a polarizer and a prism to deflect a light beam in accordance with the voltage applied to the crystal.

United States von Hundelshausen ARRANGEMENT FOR CHANGING TI'IE DIRECTIONOF POLARIZATION OF A POLARIZED BEAM OF LIGHT Ulrich Freiherr vonllundelshausen, Munich, Germany [75] Inventor:

{73] Assignee: Siemens Aktiengesellschaft, Berlin and Munich, Germany I[22] Filed: Feb. 2,1972

[21] Appl. No.: 222,773

[30] Foreign Application Priority Data Feb. 10, 1971 Germany P 21 06365.0

[56] References Cited UNITED STATES PATENTS 3,373,376 3/1968 Clark etal. 3S0/I50 '11 3,767,287 51 Oct. 23; 1973 Primary Examiner-Ronald L.Wibert Assistant Examiner-Michael J. Tokar Attorney-Carlton Hill et al.

571 ABSTRACT A method for influencing a light beam by applying anelectrical signal to cause phase modulation, amplitude modulation ordeflection of the light beam using an electro-optic crystal made of acompound having a cation and an anion and containing a thiocyanatecomplex in the anion. The compound has two different metals which areselected from Groups I and II of the periodic system of elements.Preferably one of the metals is mercury and forms with the thiocyanatecomplex the anion complex having a formula of [Hg(SCN) The other metalis preferably selected from a group consisting of zinc, cadmium, copperand gold. The electro-optic crystal is used with a polarizer to phasemodulate a light beam in accordance to the voltage applied to theelectrodes of the crystal or the electro-optic crystal can be used toamplitude modulate a light beam by being placed between a pair ofpolarizers. The electro-optic crystal can be utilized between apolarizer and a prism to deflect a light beam in accordance with thevoltage applied to the crystal.

4 Claims, 3 Drawing Figures 1 ARRANGEMENT FOR CHANGING THE DIRECTION OFPOLARIZATION OF A POLARIZED BEAM OF LIGHT BACKGROUND OF THE INVENTIONknown in the art. Such crystals are utilized to change the direction ofpolarization of a polarized beam of light such as a polarized laser beamduring amplitude modulation, or phase modulation. Electro-optic crystalshave also been utilized with a prism for the purpose of deflecting abeam of light depending on the change of polarization caused by thecrystal.

SUMMARY OF THE INVENTION The present invention is directed to anelectro-optic crystal and a method of using the crystal for influencinga beam of light in response to a voltage applied to the electrodes ofthe crystal. The electro-optic crystal comprises a compound having acation and an anion and containing a thiocyanate complex in the anion.The compound preferably includes two different metals which are selectedfrom Groups I and II of the periodic system of elements. One of themetals is preferably mercury and forms with the thiocyanate complex theanion having a formula of [Hg(SCN) The other metal is preferablyselected from a group consisting of zinc, cadmium copper and gold. Theelectro-optic crystal is used with a polarizer which polarizes lightprior to passing through the crystal which due to the direction of theapplication of a voltage to the crystal causes phase modulation in thebeam oflight or ifa second polarizer is utilized to analyze the lightpassing through the crystal can cause an amplitude modulation of a beamof light. In another embodiment of using the crystal, the crystal islocated between a polarizer and a prism which deflects the beamdepending the direction of polarization applied to the beam of light asit passes through electro-optic crystal which is being modulated by theapplication of voltage to its electrodes.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a schematic arrangement of aphase modulator utilizing an electro-optic crystal and method of thepresent invention;

FIG. 2 is a schematic arrangement of an amplitude modulator utilizingthe elctro-optic crystal and method of the present invention; and

FIG. 3 is a schematic arrangement of a light deflector utilizing theelectro-optic crystal and method of the present invention.

' DESCRIPTION OF THE PREFERRED EMBODIMENTS The principles of the presentinvention are particularly useful in providing an electro-optic crystalutilized to influence a light beam by changing the direction ofpolarization of a polarized light beam as it passes through the crystalwhich changes in polarization are dependent upon a voltage applied toelectrodes provided on the crystal.

, The crystal is made of a compound having a cation and an anion with athiocyanate complex in the anion of the compound. The compound containstwo different metals which were selected from Groups I and II of theperiodic system of elements. Preferably one of the metals is mercury andforms with the thiocyanate complex the anion complex of the compoundwhich has a formula of [I-Ig(SCN) The other metal is selected fromGroups I and II of the periodic system'of elements and preferably is ametal selected from a group consisting of zinc, cadmium, copper or goldwhich forms the cation of the compound which will have a formula such asZn[I-Ig(SCN) or Cd[l-lg(SCN)li].

Electro-optic crystals of the above mentioned compound can be obtainedfor instance from a gel in particular silica gel in which the componentsare brought to a reaction. This method enables the obtaining of largecrystals of the compound which are hardly soluble and thus will only befinally crystallized during the usual precipitation. The method forproducing the compound, for instance may include the formation of thegel, in its first step, containing one of the components of the crystal.The other reaction component is then inserted into the gel by means ofcoating the gel with an aqueous solution containing the other componentwhich solution diffuses into the gel. However, the intrusion of thereaction component can also be effected from other kinds of solutions orfrom the gas phase.

Crystals containing these compounds belong to the tctragonal crystalglass I7, and has electro-opticcoefficients, which do not disappear, ofr -r r r r -r and r The c-axis is the optical axis of the crystal andthe two axes which are vertical to the c-axis are the a-axes. I

An embodiment of the method for utilizing the electro-optic crystal forinfluencing a light beam by phase modulating is schematicallyillustrated in FIG. I. The apparatus for phase modulating a light beamcomprises a polarizer 1 arranged to polarize a light beam, such as alaser beam 3, prior to passing through an electrooptic crystal 2 made ofa compound such as zinc-mercury-thiocyanate ofa formula Zn[I-Ig( SCN) Anelectrical signal to control phase modulation applies a control voltageU to electrodes on the electro-optic crystal 2 which voltage induces anelectric field in the crystal and changes the optical characteristics ofthe crystal in response to the applied voltage to phase modulate thelight beam passing through the crystal. The largest degree of modulationof the light beam occurs when the In order to utilize a transverseelectro-optic effect of the crystal 2, the electrical field and thepolarizer l are arranged with their direction parallel to the c-axis ofthe crystal2 while the light beam hits the crystal parallel to thea-axis (electrical optical coefficients r r are utilized). Altemately,the direction of the electrical I field can be arranged to be parallelto an a-axis of the crystal 2 but vertical to the light beam, and thepolar- In order to utilize the longitudinal effect of the electro-opticcrystal 2, the light beam and electric fields are arranged with. adirection parallel to the a-a'xis of the 7 crystal, and the polarizeris' arranged with the direction of polarization at an angle of 45 to thea-axis (electrical optical coefficients r r are utilized). In analternate arrangement for utilizing the longitudinal effect of thecrystal, either the direction of the light beam or the direction of theelectrical field or both can be arranged to extend parallel to thec-axis, and the direction of polarization of the polarizer is arrangedat an angle of 45 to the c-axis of the crystal (electrical opticalcoefficient r is utilized).

The electro-optic crystal of the present invention can be utilized in amethod for amplitude modulating of a light beam in an arrangement inwhich the crystal 2 is arranged between a polarizer 1 and a secondpolarizer means such as a .polarizer 4 so that the light beam 3 passesthrough the polarizer 1, through the crystal 2, and then through thepolarizer 4. As illustrated, the polarizers l and 4 are cross polarizerswith the direction of polarization extending at right angles to eachother. Thus when a beam is polarized by a polarizer 1 into one directionand this direction of polarization is rotated by the passing of thelight beam through the electro-optic crystal 2, the second polarizer 4,which is often referred to as a polarization analyzer, will produce anamplitude modulated beam of light depending on the amount of rotation ofthe polarization of the light beam as it passes through the crystal 2.It is possible to arrange the polarizer or analyzer 4 with its directionparallel to the direction of the polarizer l and a greatest amplitudewould occur when the crystal 2 does not alter the direction ofpolarization of the light beam passing therethrough.

The highest degree of modulation in the device of FIG. 2 is obtainedwhen the polarization direction of the light beam does not coincide withthe main axis of the refraction-index ellipsoid which is induced by theelectrical field caused by the application of voltage U to theelectrodes of the crystal 2. In fact, the highest degree of modulationoccurs when the direction of polarization is at an angle of 45 to themain axis of the ellipsoid.

In order to utilize the transverse electro-optic effect electricalsignal. The arrangement is schematically il-' lustrated in FIG. 3 forpassing a lightbeam such as laser beam 3 through a polarizer 1, throughthe crystal 2 and then through a prism 5 which separates the light beaminto one of two paths depending on its direction of polarization. Forinstance the prism 5 may be a Nicol prism, a Wollaston prism or a Rochonprism, which are known in the art. The best results are obtained whenthe polarization direction ofthe light beam does not coincide with themain axis of the refraction-index ellipsoid which is induced by theelectrical field caused in the crystal by the applied voltage. In theoperation of the apparatus of FIG. 3, the application of a halfwayvoltage to the crystal 2 will cause the light beam 3 to be deflected inone of two directions by the prism 5. When no voltage is applied to thecrystal 2, the light beam will be deflected by the prism 5 in the otherdirection which direction is illustrated asan arrowed line. By arrangingseveral such crystals and prisms in series the beam of light can bedeflected in several directions with the number of directions beingequal to 2" where n is the number of crystals used.

In each of the above embodiments of the method for utilizing thecrystal, both the transverse and longitudinal electro-optic effects canbe utilized. For example, the electrical field which is applied to thecrystal for the control, can coincide with the direction of inclinationof the light beam orbe vertical to the direction of inclination of thelight beam. Also the inclination direction of the light beam can beapplied with respect to the crystal axis and the electrical field can beutilized which is applied at that inclination. However since only thestated electrical optical coefficients can be utilized, the fields mustbe derived from above described coeffi- I cients.

of the crystal 2, the light beam is arranged to be parallel to thea-axis with the electrical field being parallel to the c-axis and thedirection of polarization of the polarizer 1 and the analyzer 4 are atan angle of45 to the c-axis of the crystal (the electrical opticalcoefficients r r are utilized). Alternately, the light beam is arrangedto be parallel to the a-axis with the electrical field parallel to thea-axis but vertical to the direction of the light beam and the polarizer1 and analyzer 4 are arranged with their direction parallel to thea,-axis (electrical optical coefficients r r are utilized).

in order to utilize the longitudinal effect of the elec- .tro-opticcrystal 2, the direction of the light beam and electrical field arearranged to be either parallel to the method for deflecting a light beamin response to an Although various modifications might be suggested bythose versed in the art it should be understood that I wish to embodywithin the scope of the patent war- 'voltage to the crystal in adirection extending transverse to the path of the beam of light throughthe crystal to create an electrical field which causes rotation of thedirection of polarization of the beam of polarized light, said crystalconsisting of a thiocyanate complex.

and two different metals selected from a group consisting of mercury,zinc, cadmium, copper, and gold. one of the two metals being a cationand the other of the two metals forming with the thiocyanate complex ananion complex.

2. An arrangement according to claim 1, wherein said other of two metalsconsist of mercury and forms with the thiocyanate complex an anioncomplex having a formula of [Hg(SCN),,].

3. An arrangement according to claim 1, wherein said crystal consists ofa crystal having a formula of Cd[l-lg(SCN 4. An arrangement according toclaim 1, wherein said crystal consists of a crystal having a formula ofZn[Hg(SCN)4].

ass-sis

2. An arrangement according to claim 1, wherein said other of two metalsconsist of mercury and forms with the thiocyanate complex an anioncomplex having a formula of (Hg(SCN)4).
 3. An arrangement according toclaim 1, wherein said crystal consists of a crystal having a formula ofCd(Hg(SCN)4).
 4. An arrangement according to claim 1, wherein saidcrystal consists of a crystal having a formula of Zn(Hg(SCN)4).